Control of quorum sensing systems in lactic acid bacteria and determination of their effects on exopolysaccharide production

In this thesis study, the EPS synthesis mechanisms of LAB cultures, which have the ability to produce EPS at different levels, were controlled through QS or QQ systems, thus reducing and increasing EPS synthesis. Within the scope of the study, the extraction of QS-QQ active polyphenols from herbal sources such as rosehip, turmeric, cinnamon and mahaleb was carried out using water, ethanol, methanol, and ethanol-water. Additionally, whey (PS) has been used as a good source of oligopeptides. Minimum inhibition concentrations of the samples were determined in order to determine whether the extracts were effective by showing direct QS or QQ activity on LAB EPS synthesis. The extracts with determined minimum inhibition concentrations were added to the culture medium at 3 different rates (0.01%, 0.4%, 2%) and their effects on LAB EPS fermentation were examined. The kinetic parameters of the culture medium were determined by determining the total number of organisms, total sugar and EPS amount. As a result of the study, it was determined that cinnamon methanol, ethanol, and ethanol-water extracts significantly reduced the EPS production amounts of the Lb. namurensis MF210 strain without changing the number of living cells. In particular, %2 cinnamon ethanol-water extracts reduced the EPS production amount of Lb. namurensis MF210 strain from 1.15 g/L to 0.10 g/L, and the specific EPS production rate also decreased. In mahlep water extracts (0.4% and 2%), the EPS production amounts of the Lb. brevis MF87 strain were found to be higher than the control group (1.61 g/L), and the EPS production amounts increased to 1.97 g/L and 2.44 g/L, respectively. The positive and negative effects of such lactic starter cultures due to EPS synthesis in fermented products were indirectly controlled through the QS system. These changes, which are brought about by the control of the QS system in EPS synthesis, will not cause any change in cell proliferation and will not cause any negativity during fermentation.